1) Field of the Invention
The present invention relates to a developing device used in an electrophotographic field, wherein an electrostatic latent image is visually developed by using a one-component developer, particularly a non-magnetic type one-component developer.
2) Description of the Related Art
As is well known, an electrophotographic printer carries out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image carrying body such as an electrophotographic photoreceptor; forming an electrostatic latent image on the electrically charged surface of the electrophotographic photoreceptor by optically writing an image thereon, using a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electostatic latent image zone; electrostatically transferring the developed visible image to a paper; and fixing the transferred image on the paper. Typically, the electrophotographic photoreceptor is formed as a photosensitive drum having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof.
In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as it enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic fine carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller, provided in the vessel as a developing roller, in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed theron.
In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein.
A one-component developer is also known in this field, and a developing device using same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles). Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles composing the magnetic type one-component developer includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above.
A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Pat. No. 3,152,012 and No. 3,754,963, Japanese Examined Patent Publication (Kokoku) No. 60-12677, and Japanese Unexamined Patent Publications (Kokai) No. 62-976, No. 62-118372, No. 63-100482, and No. 63-189876. These developing devices include a vessel for holding the non-magnetic type one-component developer, and a conductive elastic roller provided within the vessel as a developing roller in such a manner that a portion of the elastic roller is exposed therefrom and can be pressed against the surface of the photosensitive drum. The conductive elastic developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material or the like. When the conductive rubber roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the conductive rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. The developing device further includes a blade member which is resiliently pressed against the surface of the developing roller, to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween.
In this developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a bias voltage applied to the conductive solid rubber developing roller.
To achieve a proper development of the latent image by the developing rubber roller, an elasticity or hardness of the developing roller is an important parameter, because the development quality and the development toner density are greatly affected by a contact or nip width between the photosensitive drum and the solid rubber developing roller pressed thereagainst. Namely, the developing roller must be pressed against the photosensitive drum so that a given nip width by which a proper development is obtained is established therebetween. When the developing roller is formed as a solid rubber roller, it may have a relatively high hardness. For example, when measured by an Asker C-type hardness meter, the solid rubber developing roller showed an Asker C-hardness of about 58.degree.. Accordingly, the solid rubber developing roller must be pressed against the photosensitive drum with a relatively high pressure to obtain the required nip width therebetween, but the higher the pressure exerted upon the photosensitive drum by the developing roller, the greater the premature wear of the drum.
It has been suggested by the inventors that the developing roller be formed of a conductive open-cell elastic material, to give it a high softness. Such a conductive open-cell elastic developing roller is constituted to prevent a penetration of the toner particles into an open-cell foam structure thereof, whereby the high softness of the developing roller can be maintained over a long period.
Conventionally, even though the developing device is in a non-operative condition, the developing roller is left as it is. Namely, even during the non-operative condition of the developing device, the developing roller is not released from the pressures exerted thereon by the photosensitive drum and the blade member. Accordingly, if the electrophotographic printer is not operated for a long time and/or if deterioration of the developing roller occurs, the developing roller is subjected to a plastic deformation.
In particular, the conductive open-cell elastic developing roller is suspectible to such a plastic deformation due to the high softness thereof. As is obvious, when the developing roller is deformed, a proper development of the latent image cannot be carried out. Also, when the pressures exerted on the developing roller are not released, the toner particles between the developing roller and the photosensitive drum and blade member may be adhered to each other, due to these pressures, and thus a poor development of the latent image may occur.
Conventionally, when the developing device is movable away from and toward the photosensitive drum, to resiliently press the developing roller against the photosensitive drum, a drive motor for the developing roller is supported by a frame structure of the electrophotographic printer, and a gear train for transmitting a rotational drive force from the drive motor to the developing roller is provided on one of the side wall portions of the developing device. The gear train is not engaged with an output gear of the drive motor until the developing device is positioned at a developing position in which the developing roller is resiliently pressed against the photosensitive drum. Nevertheless, even though the developing device is positioned at the developing position, it is still slightly movable toward and away from the photosensitive drum, and accordingly the developing device may subjected to a twist motion due to the drive force of the motor, and thus it is difficult to resiliently press the developing roller against the photosensitive drum with a uniform linear pressure, resulting in an uneven development of the latent image.
Also known is a developing device provided with a detector for detecting a lack of the developer held in the vessel and raising an alarm to inform the operator that the consumption of the developer has reached a predetermined level, as disclosed in Japanese Examined Patent Publication No. 62-502, but the conventional detector is very complex and costly.
Furthermore, a developing device provided with a seal arrangement for preventing a leakage of the developer at locations near to the ends of the developer roller is known. This leakage of the developer is apt to occur at locations near to the ends of the developer roller because, when a thickness of the developer layer is regulated by the blade member, the excess developer has a tendency to be pushed toward the ends of the developer roller. The conventional seal arrangement does not provide an effective and complete prevention of the developer leakage at the ends of the developing roller.